The present invention relates to a method for determining the path of a borehole, and more specifically to determining the position of a well borehole relative to data from a seismic survey.
More particularly, the present invention relates to a method for determining the path of a borehole which extends through subsurface formation layers; these formation layers may also be seismic boundaries and appear as reflectors in a processed seismic section (which section results from a seismic survey). As used herein, the term "seismic section " means the processed results of a seismic survey.
Usually a well path is planned based upon the position of seismic boundaries seen on a seismic section. Thus, when a borehole is drilled to reach a subsurface region for production purposes, it is of great interest to monitor the path of the borehole through said section before reaching said region. In particular, it could be necessary to correct the path by appropriately adjusting drilling conditions.
It is known from U.S. Pat. No. 2,062,151 to make subsurface determinations of the bottom of a borehole with reference to the position of the mouth of the borehole at the earth's surface. For example, the bottom position of said borehole and the seismic wave velocity are determined by solving a set of equations in accordance with a set of parameters to be determined and from travel time measurements. However, only surface related position coordinates are obtained.
From International Patent Application No. WO 88/04435 it is known to use seismic waves that originate from a drill bit while drilling; these seismic waves are received by sensors near the earth's surface, coming either directly from the drill bit or after reflection from seismic boundaries. In this manner, geological formations being drilled are also imaged. However, to process the wave data so obtained, transfer functions have to be employed; these transfer functions represent different formation conditions and may be implemented in electronic circuitry, for example, electronic filter means.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods are provided for determining the path of a borehole relative to a seismic section.